Crypto-condition Update

Hey all,

Last week, Jehan, Zaki, Evan and I met at Jehan's office to work on
cryptoconditions, specifically the conversation turned to two things a)
changing the scheme from a condition/fulfillment scheme to more of a
signature scheme and b) fleshing out the multi-message functionality.

After the conversation it turned out that the devil with multi-message is
really in the details. So what I ended up doing over the weekend is update
the spec with the change to make it act more like a signature scheme. This
turned out to be a *hugely* simplifying change and I'm very happy with it,
nice work Jehan, Zaki, Evan!

As for adding multi-message support, I now believe that it should be
out-of-scope for v1. It requires the ability to destructure objects and
directing the right parts of the signed message to the right conditions. We
should still work on it, but I think it's a very valid choice if we decide
not to include it in v1. Neither Jehan's nor Five Bells use cases require
it as a feature and it can be easily added in the future by adding a new
condition type to do the destructuring.

Note that we may also change the name of the scheme: Evan suggested
"Composite Signatures" - which is the front-runner so far. But I didn't
want to make a ton of nomenclature changes until we've all agreed on a new
set of terminology.

Here's the PR - all of the changes and rationale are described therein:

https://github.com/interledger/five-bells-condition/pull/14

- Stefan